A gear is a rotating machine part having cut teeth or cogs that engage or mesh with another toothed part, such as another gear, in order to transmit rotational power. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a toothed rack, thereby producing translational motion instead of rotation.
The earliest known reference to gears was circa A.D. 50 by Hero of Alexandria, but they can be traced back to the Greek mechanics of the Alexandrian school in the 3rd century B.C. and were greatly developed by the Greek polymath Archimedes (287-212 B.C.).
Gears made from a rigid material such as metal or metal alloys are well known and are used in many applications. Such gears may withstand high torque, but have a significant shortcoming in that they tend to be heavy and also to generate a great deal of noise when they mesh with other metal gears.
Gears of the sort used in toys and light-weight consumer appliances can be made of light-weight thermoplastic and thermosetting materials, because their loading tends to be almost trivial, as, for example, in the case of small clocks. However, modern power-transmission gears that convey tens of kilowatts and greater amounts of power find use used in stationary equipment, in ships and in cars, motorcycles and aircraft. The weight of gears tends not to be critical in stationary applications or in large ships. But in lightweight transportation systems including cars, motorcycles, bicycles and especially aircraft, weight is an important consideration.
More specifically, gears used in rotorcraft applications are designed such that the minimum weight is attained without sacrificing reliability or safety. Since the drive system is an appreciable percentage of the overall rotorcraft vehicle weight (˜10%), many approaches have been applied to improve the power to weight ratio of these components.